Braking mechanisms for stopping rotating bodies are well known in the prior art. Automatic, velocity-responsive braking systems are of particular utility in conjunction with elevator driving mechanisms. A variety of practical and legal considerations make it desirable and necessary for an elevator system to include safety braking apparatus for stopping the elevator car when some part of the system fails. To prevent injury to passengers and to minimize physical structural damage to the elevator system itself the braking system must quickly and reliably decelerate the car. Usually some type of velocity-sensitive governor mechanism is included to trip the braking apparatus in response to a predetermined overspeed condition.
One prior art brake for stopping a rotating body is disclosed in U.S. Pat. No. 1,814,841, issued to U.S. Mosleh on July 14, 1931, wherein a solenoid-actuated plunger is moved into the path of a stop arm in order to stop a rotating wheel. A prior art elevator braking concept utilizes a speedresponsive centrifugal governor to force wedge-shaped or tapered brake shoes into frictional contact with an elevator guide rail to produce controlled deceleration. The latter concept is generally disclosed in U.S. Pat. No. 3,706,361, issued to W. A. Paulssen et al on Dec. 19, 1972, U.S. Pat. No. 3,762,512, issued to R. McIntyre on Oct. 2, 1973, and U.S. Pat. No. 3,799,298, issued to W. Loomer on Mar. 26, 1974. The latter references disclose some form of centrifugal governor which actuates the braking shoes in response to an overspeed condition.
Another prior art form of elevator braking mechanism employs centrifugally actuated stop pawls which are mounted on a rotating member. As the velocity of the rotating member increases, the pawls are urged outwardly into braking contact with a usually stationary stop member. This concept is disclosed generally in U.S. Pat. No. 2,511,697, issued to W. C. Clift on Dec. 12, 1947, U.S. Pat. No. 3,695,399, issued to M. R. Laing on Oct. 3, 1972, and U.S. Pat. No. 3,729,071, issued to M. R. Laing et al on Apr. 24, 1973. The latter two patents are owned by the same assignee as in the instant case. Both of the latter two patents employ centrifugally actuable strike elements or pawls mounted on rotating brake member which are urged outwardly at a predetermined velocity to engage a stationary contact member thereby stopping the rotating body. When the pawls engage the stopping member to stop the rotating body, the resultant shock forces are dampened by the frictional braking surfaces provided by a plurality of interleaved annular driven and driving discs utilized by the brake assembly.
Centrifugal brake acuators, such as the aforementioned pawls or the like, can be disadvantageous in several respects. Centrifugal brake actuating systems will be unreliable unless they receive regular maintenance. Since they will be used so seldom, the effects of corrosion and metal fatigue will vary their response characteristics over time. It can also be very difficult to adjust the speed at which prior art centrifugal governors will actuate. The actuation speed is of course a function of physical characteristics such as length and mass which are not easily varied. Prior art centrifugally actuated pawls can also be disadvantageous because the speed at which they are triggered can vary within wide limits. Speed governors of the latter type also tend to be comparatively slow.